Hemophilia is a genetic disorder of blood coagulation affecting approximately 17,000 individuals in the United States. The two most commons forms of hemophilia are hemophilia A and hemophilia B, caused by defects or deficiencies in clotting factors VIII and IX, respectively. While treatment is effective for many people with hemophilia, it consists of life-long, intravenous infusions with clotting factor administered during or after a bleeding event. This therapy has many drawbacks, and thus gene therapy has been investigated as a means of curing hemophilia. Hemophilia is among those genetic disorders most likely to be amenable to gene therapy because it results from defects within single genes. Gene therapy for hemophilia would transfer functioning clotting factor genes into cells in a person with hemophilia, enabling that individual's body to manufacture clotting factor proteins. There has been considerable success in pre-clinical studies in using various viral vectors to obtain sustained expression of clotting factor in animals. Also, the first human trial for an ex vivo, nonviral gene therapy began in December 1998. However, a number of research questions remain unanswered, and progress in the field is facilitated by holding regularly convened workshops where investigators can discuss the current state of their work. The National Hemophilia Foundation proposes to hold the third in a series of gene therapy workshops in March 2000. The last workshop in November of 1998 raised troubling questions about toxicity and inhibitory response in animal trials, and a major portion of the workshop will be devoted to the immune response to gene therapy. Other concerns to be addressed include translation of pre-clinical findings to human trials; ethical concerns in the use of human subjects; the effect of hepatitis C and HIV infection and treatment on gene therapy; regulatory issues involved in approval of gene therapy trials; and intellectual property issues that may impede the marketing of gene therapy products. The workshop affords a critically important opportunity for open communication and debate among basic researchers, clinicians, federal regulators, representatives of biotechnology companies, and members of the bleeding disorders community as human clinical trials proceed.